Method and apparatus for the production of artificial structures



April 7, 1942. 5 w, GRAVES 2,278,875

METHOD AND APPARATUS FOR THE PRODUCTION OF ARTIFICIAL STRUCTURES Filed Sept. 29, 1938 George De 14 571 Grave; INVENTOR.

BY X ATTORNEY UNiTED STATES PATENT OFFICE METHOD AND APPARATUS FOR THE PRO- DUCTION OF ARTIFICIAL STRUCTURES George De Witt Graves, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application September 29, 1938, Serial No. 232,314

10 Claims.

The present invention relates to an improved method and apparatus for the production of artificial structures such as filaments, yarns, ribbons, sheets and the like. More particularly, this invention relates to production of such structures from filament-forming or film-forming compositions which are subject to bubble-formation under the conditions present during the extrusion of the composition. The invention has particular utility in the spinning of molten organic filament-forming compositions which are subject to bubble-formation as a result of decomposition While in their molten state.

Prior to the present invention, continuous structures, such as filaments, ribbons and sheets, have been produced by continuously extruding a viscous filamentdorming or film-forming composition through an orifice by means of a pump such as a gear pump. Gear pumps have been designed to produce the necessary pressure and at the same time function as a metering pump to deliver a substantially constant quantity of material in unit time.

Heretofore, in the extrusion of viscose and similar solutions it has been found necessary to eliminate bubbles. which are present as a result of the agitation of the mass, by placing the solution under vacuum for a period of time.

In the extrusion of certain filament-forming compositions great difficulty has been experienced due to the continuous bubble-formation in the composition under the conditions present during the extrusion thereof. This has been a particularly troublesome problem in the extrusion of molten synthetic linear polymers of the type described in the U. S. patent to Carothers No. 2,071,250. Due to decomposition of such compositions while in the molten state bubbles are continuously formed therein just prior to the extrusion thereof. In spite of all the care that can be taken in the control of such an operation there will be a very small amount of decomposition resulting in gaseous products which will be present as bubbles. The bubbles interfere with the capacity of the pump to meter uniformly. Such bubbles cannot be removed from melts by placing the melt under vacuum in the abovementicned manner, since the bubbles are formed therein under the conditions present just prior to and during the extrusion thereof. The same conditions may be present in filmor filamentforming compositions other than melts. The methods and apparatus of the present invention will, therefore, apply equally well to compositions other than melts.

It is, therefore, an object of the present invention to provide an improved method and apparatus for the extrusion of filmand filamentforming compositions which are subject to bubble-formation just prior to the extrusion thereof.

It is another object of this invention to eliminate bubbles from filmand filament-forming compositions, which compositions are subject to bubble-formation under the conditions present just prior to and during the extrusion thereof.

It is a further object of this invention to provide an improved method and apparatus for the extrusion of molten organic filmand filamentforming compositions which are subject to decomposition with bubble-formation whereby to eliminate said bubbles from said composition prior to and during the extrusion thereof.

Other objects of the invention will appear hereinafter.

The objects of the invention may be accomplished by imparting sufiicient pressure to the composition to be extruded to eliminate the bubbles from the composition, and metering said bubble-free composition, that is to say, passing the same at a substantially constant rate, for example, by means of a pump, to an extrusion device, such as a spinneret, so constructed and arranged as to maintain said composition under pressure until it i extruded.

The invention will be more easily understood by reference to the following detailed description when taken in connection with the accompanying illustration which is a diagrammatic side elevational view of a device constructed in accordance with the invention.

Referring to the illustration, container 5 is adapted to hold a quantity of solid organic filament-foiming composition. The solid, which is preferably in the form of small flakes, drops by gravity onto the melting grid 1 which is maintained at a temperature above the melting point of the composition. A the solid composition melts it passes, in the form of droplets 9, to a small reservoir Ii. The composition as a result of decomposition in its molten state forms very small bubbles i3. The bubble-containing composition is compressed by means of a gear pump l5 to a pressure sufiiciently high to eliminate the bubbles as well as to prevent further bubbleformation. The bubble-free composition is passed to a small reservoir I! which may be little more than an interconnecting conduit between pumps. The molten composition is metered by means of gear pump l9 from reservoir I! to a filter pack 2| which is composed of a plurality of fine screens which function to filter the composition and maintain the composition under pressure until it reaches the spinneret 23 and is extruded in the form of filaments.

The gear pump l5 must have a greater capacity than pump l9 so that the intermediate reservoir I! will contain the molten composition under considerable pressure to maintain the same free of bubbles. The metering pump 19 must always pump substantially bubble-free composition in order to continuously pass a substantially constant amount of composition per unit of time.

If necessary a by-pass conduit 25 may be provided between reservoir l1 and reservoir I. This by-pass conduit may contain a pressure relief valve 21. By this construction a quantity of the molten composition may be continuously, or discontinuously, returned to reservoir H whenever the pressure reaches a predetermined figure, and thereby prevent building up of an excessive pressure between the two sets of gear pumps. If desired, of course, a by-pass such as 25 can be eliminated by employing a pump having sufficient slip or back-pressure leakage to prevent the building up of objectionable pressure.

The melting of the solid filmor filamentforming material may be carried out at any desired pressure, for example, at atmospheric pressure or in the presence of a few pounds gas pressure or below atmospheric pressure if the flow to the pump inlet is assured by the apparatus design or the low viscosity of the filmor filament-forming composition. The first of the two pumps, 1. e., the pump imparting a pressure to the filament-forming composition to eliminate bubbles therefrom is operated to impart a sufficiently high pressure to the filament-forming composition to eliminate bubbles therefrom, and also to eliminate bubble-formation. A pressure of at least 50 pounds per square inch must usually be imparted to the composition being extruded and possibly also depends upon other conditions such as speed of extrusion, viscosity of the composition, temperature of composition, rate of gas resolution, etc. As a general rule the pressureimparting pump is operated to impart the pres sure to the composition somewhat higher than necessary since it has been found that an excess pressure will have no undesirable influence on the extrusion of the material. The following detailed example is submitted to illustrate the process with reference to a special compound. It is to be understood, however, that the invention is not limited to the specific details set forth in this example.

Example I Polyhexamethylene adipamide (melting point about 263 C.) was melted at a temperature of about 280 C. and under 10 pounds nitrogen pressure was fed to the inlet port of a pressure gear pump such as illustrated by I5 in the drawing. The gear pump imparted a pressure of approximately 500 pounds per square inch to the molten composition which, prior to 1 :ing placed under pressure, had a tendency to decompose and form bubbles. The molten material under pressure contained substantially no bubbles, and no bubble-formation was evident. The molten composition was then pumped, by means of a metering pump, such as illustrated by 19 in the drawing, into a screen pack containing a large plurality of screens of varying mesh sizes to prevent any rapid and substantial drop in pressure. The pressure on the molten composition then forced the same through the screen pack and through a spinneret,for the production of filaments. The molten composition passing from the metering pump l9 was substantially free from bubbles and the pressure measured at a point between the screen pack and the metering pump was substantially constant at 595 pounds per square inch. The resultant threads had a spread in denier of only 6.1 per cent, and the standard percentage deviation was 1.08 per cent. In similar trials the spread was maintained as low as 3 per cent and the standard deviation 0.8 per cent.

The filter pack between the metering pump and the spinneret was composed of a large number of screens of the following mesh sizes (the meshes referred to are stated in terms of standard square mesh sieve screens): five l6-mesh screens, three 30-mesh screens, five -mesh screens, thirteen -mesh screens, one hundred twenty-five ZOO-mesh screens and twenty-nine 325-mesh screens. The filter pack composed of such a number of screens has been found in practice to offer at least as much over-all resistance to the liquid flow as the spinneret holes themselves. That is to say, the pressure drop through the filter pack is at least equal to the difference in pressure between the inside and outside of the plate in which the spinning holes are drilled. This, however, is not considered to be a critical condition. Desirable results are obtained when the pressure drop through the filter pack is somewhat greater, as Well as somewhat smaller, than that through the spinning orifices in the spinneret.

The denier uniformity of the yarn was evaluated by cutting pairs of 9 cm. lengths of yarn from each of 25 successive meter lengths of yarn. The deniers calculated from the weight of these 9 cm. sections of yarn were averaged and a standard percentage deviation calculated from the individual deviations. The standard deviation is the root mean square of the deviations from the mean. The standard deviation was divided by th average denier and expressed in per cent.

In the above-mentioned example when, under the same conditions, the molten polyhexamethylene adipamide is forced through the spinneret by means of a single gear pump the spread in denier of the yarn produced was found to be 36 per cent and the standard deviation from the mean was found to be 5.3 per cent.

In the foregoing description the invention has been illustrated with specific reference to the spinning of poiyhexamethylene adipamide, a synthetic linear polyamide. The invention is obviously applicable to the melt spinning of any organic filament-forming composition, however, which is subject to the formation of bubbles, due to whatever cause, under the conditions just prior to and during the spinning thereof provided the gases redissolve under pressure and the decomposition is slow enough to permit metering before sufficient gases are formed to exceed the solubility under the conditions of temperature and pressure. As examples of such filament-forming compositions in which bubbledormaticn may be present, the following may be mentioned: Synthetic linear polyamides, that is, synthetic linear polymers containing CONH- units in the linear chain, synthetic linear polymers such as polyesters, polyethers, polyacetals and mixed polyester-polyamides such as may be prepared by condensation reactions as described in U. S.

Patent No. 2,071,250 may also present problems of bubble-formation which can be remedied by the process of the present invention. Other types of synthetic polymers such as ethylene polymers, vinyl polymers, polystyrene and polyacrylic acid derivatives may also be spun with advantage in accordance with the present invention.

The filament-forming material used in accordance with the present invention may contain modifying agents, e. g., luster-modifying agents, plasticizers, pigments and dyes, antioxidants, resins, etc. The present invention can be used to advantage in extruding filmor filament-forming compositions in which the bubbleformation is caused by the presence of a modifying agent.

The means for imparting pressure to the filament-forming composition for the elimination or prevention of bubble-formation may comprise gear pumps, screw pumps, piston pumps, or, in fact, any other type of means for imparting pressure to the filament-forming composition including the application of high gas pressures using gases which are preferably inert to the filamentforming compositions and of limited solubility in them, e. g., oxygen free N2 or H2. The means for metering the filament-forming composition and forcing the same through a spinneret may similarly consist of any type of metering pump or any other means whereby the composition can be forced through the spinneret at a substan-- tially constant rate. It is only necessary that the first of the two means arranged in series function to impart sufficient pressure to the filament-forming composition to eliminate bubbles and bubble-formation so that the following means may function properly to meter a substantially constant amount of the filament-forming composition to the spinneret per unit of time.

Filters to remove foreign particles may, if desired, be included between the two pumps or on either side of the said pumps.

The method and apparatus of the present invention have been found to give a satisfactory solution to the spinning of compositions having a tendency to form bubbles during the extrusion thereof. Such compositions may be spun in accordance with the present invention with an increased denier uniformity of filaments and yarn. Filaments and yarns produced in accordance with this invention will also be free from bubbles in their solidified state.

and comprising a material which is subject to decompositionwith bubble-formation, placing the composition containing bubbles under a pressure of at least 50 pounds per square inch whereby to substantially eliminate said bubbles, metering said bubble-free composition to an extrusion means, and extruding said bubble-free composition, said composition, during the above-mentioned steps, being maintained under sufiicient pressure to prevent reformation of bubbles in said composition.

3. In a process for the extrusion of a molten structure-forming composition containing bubbles and comprising a material which is subject to decomposition with bubble-formation, placing and maintaining the composition containing bubbles under pressure of at least 50 pounds per square inch to substantially eliminate said bubbles, metering said bubble-free composition to Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited except as set forth in the appended claims.

I claim:

1. In a process for the extrusion of a molten structure-forming composition containing bubbles and comprising a material which is subject to decomposition with bubble-formation, placing the composition containing bubbles under sufficient pressure to substantially eliminate said bubbles, metering said bubble-free composition to an extrusion means, and extruding said bubble-free composition, said composition, during the abovementioned steps, being maintained under sufficient pressure to prevent reformation of bubbles in said composition.

2. In a process for the extrusion of a molten structure-forming composition containing bubbles an extrusion means while maintaining the composition under a pressure of at least pounds per square inch, and extruding said bubble-free composition while maintaining the same under a pressure of at least 50 pounds per square inch to prevent reformation of bubbles in said com- Position.

4. In a process as defined in claim 2 in which the molten structure-forming composition comprises a synthetic linear polymer.

5. In a process as defined in claim 2 in which the molten structure-forming composition comprises a synthetic linear polyamide.

6. In a process as defined in claim 2 in which the molten structure-forming composition comprises polyhexamethylene adipamide.

'7. In an apparatus for the extrusion of a molten structure-forming composition which is subject to bubble-formation under the conditions prior to and during the extrusion thereof, means for imparting a pressure to said composition whereby to substantially eliminate the bubbles therefrom, an extrusion element containing orifices, metering means for forcing said composition at a substantially uniform rate to said extrusion element while maintaining the same under sufficient pressure to prevent reformation of bubbles in said composition, and filter pack means on the interior of said extrusion element in contact with the interior surface of the extrusion element containing said orifices for the prevention of rapid drop in pressure on said composition.

8. In an apparatus for the extrusion of a molten structure-forming composition which is subject to bubble-formation under the conditions prior to and during the extrusion thereof, means for imparting a pressure to said composition whereby to substantially eliminate the bubbles therefrom, an extrusion element containing orifices, metering means for forcing said composition at a substantially uniform rate to said extrusion element while maintaining the same under sufficient pressure to prevent reformation of bubbles in said composition, and screen pack means on the interior of said extrusion element in contact with the interior surface of said element containing the said orifices for the prevention of rapid drop in pressure on said composition.

9. In a process for the extrusion of a molten structure-forming composition comprising a synthetic linear polymer which, in the molten state, is subject to continuous decomposition and formation of gaseous decomposition products, placing the molten composition under suflicient pressure to dissolve said gaseous decomposition products, metering said composition to an extrusion means, and extruding said composition, said composition, during the above-mentioned steps, being maintained under sufficient pressure to maintain said gaseous decomposition products dissolved in said composition.

10. In an apparatus for the extrusion of a molten structure-forming composition which is subject to bubble-formation under the conditions prior to and during the extrusion thereof, gear 1 pump means for imparting a pressure to said composition whereby to substantially eliminate CERTIFICATE OF CORRECTION.

Patent No. 2,278,875-

April 7, 19h;.

GEORGE DE WI'IT GRAVES.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows; 0nd column, lines 27, 50 and 55, claims a, claim reference numeral "2 read -l--;

Page 5, sec- 5 and 6 respectively, for the and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 26th day of May,

(Seal) A. D. 19u2.

Henry Van Arsdale,

Acting Commissioner of Patents. 

